There is a continuing trend in the semiconductor industry to decrease the size of transistors in order to increase the performance and speed of a large-scale integrated circuit (LSI). Correspondingly, efforts to increase operation speeds of metal oxide semiconductor type field effect transistors (MOSFETs) include making the gate insulating film (typically containing silicon dioxide) thinner. For example, silicon dioxide gate insulating films with a thickness of 2 nm are available. While the relatively thin gate insulating films increase operation speeds, other problems are undesirably created.
In MOSFETs having a relatively thin gate insulating film, the amount of gate leakage current is deleteriously high, due to tunneling current. Gate leakage inhibits the performance of a microelectronic device. Power consumption of devices employing thin gate insulating film MOSFETs is an important concern due to gate leakage. In electronic devices, it is typically desirable to reduce the amount of power that is consumed by a microelectronic device. This is because in battery powered electronic devices it is typically desirable to reduce the amount of power consumed by the microelectronic device in order to extend the time the electrical device may be powered by a battery.
Attempts to address the gate leakage problem include use of high-K gate dielectrics, such as hafnium oxide. However, use of high-K gate dielectrics raises certain issues including mobility degradation, introduction of a high number of traps (thereby decreasing reliability), and relatively high noise (1/f noise) compared to silicon dioxide. Mobility degradation leads to poor transistor drive current. Moreover, mobility degradation contravenes an important scaling goal of improved mobility. The degradation of mobility can at least be partially attributable to localized charge traps.
Another issue with high-K gate dielectrics, such as hafnium oxide, is the difficulty associated with patterning. In many instances, a strong acid is required. The strong acid desirably removes portions of the high-K dielectric, but undesirably removes many other semiconductor device materials including silicon dioxide. Since silicon dioxide is extensively used in microelectronic devices, use of strong acids is disfavored. Consequently, to avoid serious damage the use of strong acids is avoided.